1. Field of the Invention
This invention broadly relates to a mill blank assembly used in the field of dentistry to create an inlay, onlay, crown, veneer, coping, bridge, bridge framework, implant, implant abutment or other restoration or restoration component. More specifically, the present invention is directed to a mill blank assembly that is especially adapted for use with computer-aided design and machining processes to create a dental prosthesis.
2. Description of the Related Art
A variety of dental procedures are known for replacing or repairing damaged, weakened or missing tooth structures. For example, a dental prosthesis commonly known as a filling is often used to fill cavities in teeth caused by tooth decay or caries. Somewhat larger prosthetics also used to fill cavities are known as inlays and onlays. Fillings, inlays and onlays may also be utilized to restore the shape of teeth that have been chipped or broken.
Other types of dental prosthetics include bridges, full crowns and partial crowns. Typically, these prosthetics are much larger than fillings and as a result are often more visible in the oral cavity. Full and partial crowns may be supported by remaining portions of the original tooth structure and/or by a post extending toward the bony region of the jaw. Bridges, on the other hand, are structures that connect to adjacent tooth structure and provide an artificial tooth or tooth crown to replace corresponding, missing structure.
In the past, fillings and some inlays and onlays were often made of a silver-colored metal alloy known as amalgam due to its relatively long life and relatively low cost. Another advantage offered by amalgam is that it allows a dental practitioner to fit and fabricate the restoration during a single session with a patient. Unfortunately, amalgam is not considered aesthetic since its silver color sharply contrasts to the appearance of natural teeth in the oral cavity.
Another material used for dental prosthetics, and particularly for larger inlays and fillings, is gold. However, like amalgam, the color of gold sharply contrasts with the appearance of natural teeth and is highly visible in the oral cavity. In addition, gold is relatively expensive in comparison to other dental materials.
As a consequence, many dental practitioners are increasingly turning to ceramic or polymer-ceramic composite materials for use to make dental prosthetics. Dental ceramic materials and dental polymer-ceramic composite materials can provide an appearance that closely matches the appearance of natural teeth. Such materials are also available in various color shades so that the practitioner can select a color that closely matches the color of adjacent tooth structure.
Dental polymer-ceramic composite materials for use as restoratives are available from various manufacturers in paste-type form. Such materials are often supplied in capsules that are releasably received in a receptacle of a hand-held dispenser. The dispenser typically includes a lever that, when depressed, extrudes a quantity of the material from the capsule and directly onto the tooth structure. The material includes a polymerization initiator that serves to harden the material once it has been placed on the tooth structure and shaped by the practitioner to resemble natural tooth structure.
A variety of techniques may be employed to help shape the unhardened restorative paste to a desired configuration once dispensed onto the patient""s tooth structure. For example, if the material is used to fill a relatively small cavity, the material can be dispensed directly into the cavity and then shaped by hand. A hand instrument such as a dental pick is used to help pack the material in the cavity and to blend the external surface of the paste with adjacent, external portions of the patient""s tooth. As another example, if a portion of one or more sides of a tooth is to be restored, the practitioner may elect to use a matrix band or sectional matrix band next to the tooth structure to help hold the material in place while it hardens. The matrix band or sectional matrix band serves as a formwork, similar to formwork used in concrete, to help hold the material in place and also to help define an outer surface of the composite material while it hardens.
However, larger prosthetics are often fabricated outside of the oral cavity and then placed in the patient""s oral cavity once completed. For these types of prosthetics, an impression is often taken of the patient""s tooth structure of interest along with adjacent regions of the gingiva, using an elastomeric impression material that provides a negative physical image of the tooth structure and gingival region. Next, a cast positive model is made by pouring a quantity of plaster of Paris into the impression and allowing the plaster of Paris to harden. The resulting plaster of Paris or xe2x80x9cstonexe2x80x9d model is then used in the laboratory to make a prosthetic that is ultimately transferred to the patient""s oral cavity.
The laboratory procedure for making the prosthetic may be somewhat involved, depending on the type of prosthetic that is needed. In one method, for example, a wax replica of the desired crown is built on the stone model. The wax replica is then embedded in a refractory investment material and fired to create another negative physical image of the oral structure of interest. Porcelain is then forced into the investment material under pressure and heat in order to make the crown.
However, a number of disadvantages arise when the foregoing procedure is followed to make a crown. In such a procedure, the patient typically travels to the practitioner""s office two times: a first time to enable an impression to be taken, and a second time a few days later after the stone model has been made and the crown has been fabricated in the dental laboratory. Moreover, if the completed crown must be returned to the laboratory because its shape, fit or appearance is not satisfactory, the patient is often then required to return to the dental office for a third visit. In many dental practices, the crown is not made in a laboratory that is part of the office but is instead sent to a central laboratory in another area of the town or region.
Furthermore, the fabrication of custom dental crowns and other prosthetics by hand from stone models is an art that involves a high degree of skill and craftsmanship, as well as intensive labor. Moreover, prosthetics that are placed in the anterior regions of the patient""s oral cavity are often highly visible. It is widely considered difficult to make a porcelain prosthetic that exactly matches the translucency and color of natural teeth.
Recently, increased interest has been directed toward the use of computer automated machinery for fabricating dental prosthetics, using far less labor than prior methods such as the method for making a crown described above. For example, several systems are known for collecting a set of electronic data that is representative of the patient""s tooth structure of interest. The data is then used by an automated mechanical milling machine (such as computer-aided milling machine) to fabricate a prosthetic that, when completed, closely matches the shape of natural tooth structure.
Examples of computer-aided milling machines used in the field of dentistry include the CEREC 2(trademark) and CEREC 3(trademark) machines available from Sirona Dental Systems of Bensheim, Germany, the VITA CELAY(trademark) machine from Vita Zahn Fabrik of Bad Sackingen, Germany, PRO-CAM(trademark) from Intra-Tech Dental Products, of Dallas, Tex. and PROCERA ALL CERAM(trademark) from Nobel Biocare USA of Westmont, Ill. U.S. Pat. Nos. 4,837,732, 4,776,704 and 4,575,805, as well as PCT patent application No. WO 96/37163 also disclose systems for making dental prosthetics using computer-aided milling machines.
The fabrication of a dental prosthesis using a computer-aided machining system typically involves the use of a xe2x80x9cmill blankxe2x80x9d, a block of material from which the prosthetic is cut. Dental mill blocks are often made of a ceramic material. Commercially available dental mill blanks include VITA CELAY(trademark) porcelain blanks from Vita Zahn Fabrik, VITA NCERAM(trademark) ceramic blanks from Vita Zahn Fabrik, MACOR(trademark) micaceous ceramic blanks from Corning, and DICOR(trademark) micaceous ceramic blanks from Dentsply. A dental mill blank made of a ceramic silica material as described in U.S. Pat. No. 4,615,678. An improved ceramic dental mill blank is described in applicant""s co-pending application entitled xe2x80x9cCERAMIC DENTAL MILL BLANKSxe2x80x9d, U.S. Ser. No. 09/383,560, filed Aug. 26, 1999.
Dental mill blanks may also be made of resinous materials. An example of a dental mill blank made of a polymeric resin and a filler is described in applicant""s co-pending U.S. patent application entitled xe2x80x9cDENTAL MILL BLANKSxe2x80x9d, U.S. Ser. No. 09/227,230, filed Jan. 8, 1999. Dental mill blanks made of such material exhibit superior milling characteristics such as hardness and cutting properties relative to previously known dental mill blanks.
Many commercially available dental mill blanks are made of a two-piece construction that comprises a support stub section and a milling blank section. The support section is cylindrical and adapted to fit into a collet or a Jacobs chuck of a milling machine. Often, the support section is made of metal, since the support section is ultimately detached from the milling section and does not form part of the finished prosthetic. The support section is typically made of a relatively soft metallic material such as an aluminum alloy that is easy to machine to precise tolerances.
The milling section of conventional two-piece dental mill blank assemblies is often made of one of the aesthetically-pleasing restorative materials described above so that the resulting prosthetic provides a natural appearance once placed in the oral cavity. The milling section has a flat face that is joined to a flat face of the support section by an adhesive. An example of one type of two-piece construction is described in U.S. Pat. No. 4,615,678.
It has been observed, however, that dental mill blank assemblies occasionally fracture during the milling process. In some instances, the fracture occurs in the joint between the support stub section and the milling section. It is suspected that lateral forces exerted by the milling tool on the milling section create a shear force that exceeds the strength of the adhesive bond of the joint.
Unfortunately, if the milling section has broken away from the support section before the milling process has been completed, the mill blank assembly must be discarded and replaced with a new assembly. Consequently, the fracture of dental mill blank assemblies represents a time-wasting nuisance to the personnel operating the milling system. Replacement of the dental mill blank assembly with a new assembly also represents an additional cost to the dental laboratory, the dental practitioner and the patient that is best avoided if at all possible.
The present invention is directed toward a dental mill blank assembly that presents an enhanced resistance to fracture during the time that the mill blank assembly is machined in a milling system. The mill blank assembly is especially adapted to safely resist forces exerted by a milling tool in lateral directions so that the dental prosthetic can be milled to completion. As a result, unintentional detachment of the support section from the milling section is avoided.
In more detail, the present invention in one aspect is directed toward a mill blank assembly for a dental prosthesis, and comprises a milling section made of a material suitable for making a dental prosthesis. The mill blank assembly also comprises a support section having a shaft for releasably supporting the mill blank assembly in a milling machine. The support section is fixed to the milling section and is made of a material different than the material of the milling section. One of the milling section and the support section includes a projection, and the other of the milling section and the support section includes a recess that receives the projection.
A number of additional features are also possible. For example, the mill blank assembly may include an additional one or more projections, each of which is received in an additional, respective recess. The projections and the recesses may have closely complemental cross-sectional configurations that present a precise mating fit. Additionally, an adhesive may be provided to enhance the bond between the support section and the milling section.
Another aspect of the present invention is directed toward making a dental mill blank assembly. The method comprises the acts of providing a mold having an inlet channel and a mold cavity in communication with the inlet channel, and directing a quantity of restorative material along a path that leads through the channel and into the mold cavity. The method also includes the act of hardening restorative material located in the mold cavity as well as restorative material located in the channel. The method further includes the acts of removing the hardened restorative material from the mold cavity and the channel, and coupling the hardened restorative material to a support section. The act of coupling the hardened restorative material to a support section includes the act of inserting a portion of the hardened restorative material that was formerly in the channel into a recess of the support section.
These and other features of the invention are described in detail below and are illustrated in the accompanying drawings.